As an alternative to Public Switched Telephone Network (PSTN) systems, cellular phone networks have proliferated over the last decade where users with cellular phones have access to one or more networks at almost any location. Also a recent development is the wide spread use of Voice over IP (VOIP) telephony, which uses internet protocol over wired and wireless networks. With the availability of such diverse types of communication networks and devices capable of taking advantage of various features of these networks, a new technology is bringing different communication networks together providing until now unavailable functionality such as combining various modes of communication (e.g. instant messaging, voice calls, video communications, etc.). This technology is referred to as unified communications (UC). A network of servers manages end devices capable of handling a wide range of functionality and communication while facilitating communications between the more modern unified communication network devices and other networks (e.g. PSTN, cellular, etc.).
In addition to accommodating various modes of communication, modern real time communication systems such as UC systems can facilitate presence and routing capabilities, which basically enable a user to place and receive calls (in any communication form) through a number of end devices regardless of physical location. For example, a user may define a destination number (identifier), where he/she can be reached at different times or depending on the context of the call, and the system is capable of routing the call based on the user provided rules. To accommodate such complex functionalities, modern systems typically include a large number of client devices, servers, and other specialized devices (endpoints) and software.
In order to provide satisfactory user experience, a real time communication system needs to offer accurate presence and routing fidelity. Presence fidelity means the presence state of an endpoint is reported very accurately to the user's subscribers who have subscribed for the user's presence. This applies to both users subscribing to other users and the roster information communicated in a conferencing system. The ability of the system to accurately pick and route to a valid, available endpoint is called as routing fidelity. To a large extent, routing fidelity is influenced by presence fidelity—bad presence fidelity reduces routing fidelity and consequently the number of failed call attempts due to stale endpoints becomes high.